For an interesting and well done plot twist, see Loki's "death" in Thor The Dark World
For something daft and annoying, see the "Mandarin is an actor" reveal in IM3

My guess is that Loki has either managed to place Odin into a forced Odin Sleep or that Loki did indeed die and went to Hel. Whereupon he strikes a deal with Hela, to have Odin take Loki's place in Hel.

My preference is for Hela to have some involvement in Loki's return and rise to the throne.

This... Once Loki said " I'll see you in ___". Since Asgardians wouldn't know anything aboout "HELL" I immediately thought of "HEL" and Loki has been known to have a connection with Hela often times, it didn't seem that big of a stretch in my mind that it would have something to do with his cheating death.